Chapter 18 Injection Nozzles 371 Copyright Goodheart-Willcox Co., Inc. solid matter in the fuel stream. The force of the fuel moving through the nozzle opening keeps it clear. However, this type of nozzle is prone to fuel leakage or “dribble” through the tip. Injector dribble is a major cause of preignition (knocking) in diesel engines. As fuel runs down and remains on the tip of the nozzle, carbon will form in this area and cause afterburning. Leakage or dribble obviously leads to poor fuel economy and black exhaust smoke. For these rea- sons, open injector nozzles are not frequently used. The valve used in closed nozzle injectors eliminates fuel leakage or dribble through the tip. Preignition and afterburning are all but eliminated and fuel economy improves. Modern filtration technology has also reduced clogging of nozzles by removing solids from the fuel before it reaches the nozzle. Closed type injector nozzles are the most popular nozzle type in use today. There are two types of closed nozzles, the hole type and the pintle type. Both types have a valve and seat so the fuel lines are completely closed off when no fuel is being injected. Hole Injector Nozzles Direct injection, open combustion chamber diesel engines, such as those used in heavy-duty truck applications, mainly use long-stem hole injector nozzles, Figure 18-3. The extended small diameter tip of the long-stem nozzle reduces the amount of mounting space required between cylinder head valves. In this design, the valve guide is located away from the combustion chamber and within the cooling area of the cylinder head. A typical hole nozzle consists of a valve and body that are fitted together to form a mated assembly. Since the valve and body are mated during manufacturing, valves must never be interchanged between nozzle holders. Each hole nozzle is engineered to meet the particular needs of the engine it is serving. Hole Nozzle Fuel Flow Fuel under high pressure is generated by the fuel injection pump, flows through the fuel duct in the body, and enters the pressure chamber. When the fuel pressure exerts sufficient force on the differential surface of the valve to overcome the opposing spring preload, the valve is lifted off its seat, allowing fuel to enter the sac beneath the seat. From the sac, it is directed through the spray holes into the combustion chamber, Figure 18-3. The fuel is atomized as it exits the spray holes and enters the combus- tion chamber. A hole nozzle has one or more spray holes or orifices. These openings are straight round holes through the top of the nozzle body beneath the valve seat. At the end of the pump’s injection stroke, there is a sudden drop in line pressure that results in a rapid pressure drop in the nozzle chamber. Since the pressure adjusting spring is exerting a downward force on the valve and is no longer opposed by fuel pressure in the chamber, the valve immediately reseats itself in the nozzle body, closing the nozzle. A slight amount of controlled clearance between the nozzle valve major outside diameter and nozzle body allows a small amount of leakage past the valve into the adjusting spring chamber and nozzle holder body. This leakage provides lubrication for the valve spring seat and adjusting spring. A number of design factors affect the operation and efficiency of hole type nozzles. Spray Hole Design The number, diameter, and position of the spray holes must be chosen to provide good atomization of the fuel entering the combustion chamber, Figure 18-4. This provides optimum air-fuel mixing throughout the engine’s full operating range. Body Needle Pressure chamber SAC Spray hole Cone angle of spray holes Blind hole Pressure pin Feed passage Figure 18-3. Long-stem hole injector nozzles require less mounting space between cylinder heads. A B C Figure 18-4. Configuration of spray holes or orifices in typical hole nozzles. A—Seat hole nozzle. B—Hole nozzle with con- ical fuel sac. C—Hole nozzle with cylindrical fuel sac.